1. Field of the Invention
This invention relates to the field of digital printing and imaging, and more particularly to a system and method for reproducing digitized documents such as books, periodicals, and other written materials.
2. Description of the Related Art
One important application of digital imaging technology is the conversion of books and printed material into an electronic form suitable for reading, archival, and transmittal. An example of such an application is printing books-on-demand, a new technology that is revolutionizing the book printing and publishing industry. Instead of printing thousands of copies of a book and then storing and distributing each copy, publishers can create a digital version of the book and print an exact number of desired copies. This solution is highly desirable because it eliminates inventory costs and the need to forecast expected demand for a book. Furthermore, books can never go out of print using this technology as their digital versions can persist indefinitely.
Presently, a significant number of books exist for which there is no available digital version. For reading and archival purposes, these books must be converted to digital form using a scanning process. Printed documents usually consist of text regions and image regions. The image regions are printed by converting continuous-tone originals into halftone images using a screening process. An overview of several conventional screening (or halftoning) processes is provided in the text, R. Ulichney, “Digital Halftoning,” MIT Press, Cambridge, Mass., 1987.
In order to recover the original continuous-tone image from the scanned halftone image, a process known as de-screening (or inverse halftoning) is used. A number of conventional gray-scale, halftone de-screening techniques are disclosed, for example, in Jaimes et al., “Segmentation and Automatic Descreening of Scanned Documents,” SPIE, Volume 3648, pages 517-518, 1999, and Luo et al., “A Robust Technique for Image De-Screening Based on the Wavelet Transform,” IEEE Transaction on Signal Processing, Volume 46, No. 4, pages 1179-1184, 1998.
If scanned halftoned images are printed without de-screening, moire patterns may be produced which degrade the quality of the reproduced document. Even though moire patterns are less likely to appear in other printing processes (e.g., error-diffusion halftone and stochastic halftoning algorithms), transforming a halftone image into a continuous-tone image is desirable for future image manipulation processes such as image compression and scaling. A common technique, therefore, is to segment a document into text and halftone regions, respectively, and apply a de-screening technique to the halftone regions. See, for example, Dunn et al., “Extracting Halftones from Printed Documents Using Texture Analysis,” Optical Engineering, Volume 36, No. 4, pages 1044-1052, 1997. De-screening is therefore desirable because it prevents moire patterns from appearing in the reproduced halftone image.
The most direct de-screening approach is to apply a low-pass filter to the halftone image. See, for example, Hein et al., “Halftone to Continuous-Tone Conversion of Error-Diffusion Coded Images,” IEEE Transactions on Image Processing, Volume 4, No. 2, 1995. This approach is designed for the error-diffusion halftoning process which does not employ a screening process and thus is difficult to apply for de-screening images generated by repetitive screens.
Another approach to de-screening, known as the wavelet-based approach, has been said to be applicable regardless of the screening (halftoning) process used. However, because wavelet algorithms involve down-sampling and up-sampling steps, moire patterns might appear which would inevitably deteriorate the quality of the reconstructed image. The wavelet method has not been successfully tested on halftone samples generated by screening processes.
Techniques for de-screening color halftone images are known in the prior art. These techniques typically use a smoothing filter, such as a Gaussian blur, to convert a halftone image into a continuous-tone image. A shortcoming of such filtering is that the blur not only removes the screen, but also degrades desirable image properties such as sharpness of edges and image detail.
In an attempt to overcome these two conflicting goals, conventional systems have employed an approach based on anisotropic diffusion and total variation minimization to generate piecewise smooth gray scale images. See, for example, Blomgren et al., “Total Variation Image Restoration: Numerical Methods and Extension,” IEEE Image Processing, Volume 3, pages 384-387, 1997. This approach, however, has proven unacceptable in many instances. Attempts to generalize these to color spaces have shown unsatisfactory performance.
More specifically, one of the problems with this approach is that a processed image becomes near-graphic in the sense that the image appears to have posterization effects, where there are large areas of uniform color. Although edge information is preserved, and even enhanced, the visual quality is often objectionable. To improve visual quality, many prefer the slightly blurred image produced by the blindly applied low-pass filter.
A need therefore exists for an improved method for de-screening halftone images, and more specifically a method which converts a halftone image into a continuous-tone image that exhibits less degradation of edges and image detail.